To me it looks like nutrient burn not light burn, which would cause the leaves to go brown and crispy.
18/6 is much more plant friendly and a 100 watt light would have to be almost touching the plant to cause burning.
I would be either reducing the feedings or the nutrient strength and would be quite happy to have the light at 100%, it is only 100 watts after all.
Also I would be ensuring there is run off each irrigation, this will help slow the build up of nutrient salts in the substrate.
Hope this helps,..........
Organoman.